Carrier aggregation is a combination of two or more cells or component carriers (CCs) operating at different frequencies in order to provide a broader transmission bandwidth for a mobile terminal. Depending upon its capabilities, a mobile terminal may simultaneously receive or transmit on one or more of the cells. The cells aggregated in accordance with carrier aggregation include a primary cell and one or more secondary cells. The primary cell is the component carrier that: (i) operates on a primary carrier in which the mobile terminal either performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or (ii) was indicated as the primary cell in a handover procedure. Conversely, a secondary cell is a component carrier, operating on a secondary carrier, which may be configured once radio resource control (RRC) is established and which may be used to provide additional radio resources.
Although the focus to date has been principally upon frequency division duplex (FDD) networks, TDD networks that support carrier aggregation must also be considered. Indeed, in a TDD network, the primary cell and the secondary cells may have respective TDD UL/DL subframe configurations. In Long Term Evolution (LTE) release 10, mobile terminals that support TDD signaling are required to operate in accordance with a TDD UL/DL subframe configuration that is aligned and consistent across the primary and secondary cells that are to be aggregated. Additionally, LTE release 10 required that common discontinuous reception (DRX) parameters be utilized for each of the primary and secondary cells such that the active time and the DRX pattern would be the same for each of the aggregated cells.
For mobile terminals configured in accordance with LTE release 11, however, the primary and secondary cells are permitted to have different TDD UL/DL subframe configurations. In this regard, primary and secondary cells having different TDD UL/DL subframe configurations may provide different amounts of resources and a different DL/UL ratio for the mobile terminal. For example, the use of different TDD UL/DL subframe configurations by the primary and secondary cells in LTE release 11 may permit different ones of the cells to provide different coverage by, for example, enabling more UL subframes in a lower frequency cell to enlarge the coverage. The different TDD UL/DL subframe configurations may also support inter-band carrier aggregation and co-existence with other systems and certain frequency bands. The ability to provide different amounts of resources and a different DL/UL ratio may be of particular importance to a mobile terminal that utilizes carrier aggregation since carrier aggregation is oftentimes utilized in an instance in which the mobile terminal has a relatively large amount of data to transmit, thereby increasing the importance of defining the TDD UL/DL subframe configurations of the primary and secondary cells so as to provide a suitable DL/UL ratio and to otherwise efficiently utilize the communication resources.
By allowing the primary and secondary cells to have different TDD UL/DL subframe configurations, however, the different TDD UL/DL subframe configurations of the primary and secondary cells may have overlapped subframes in some instances, such as by one of the cells having a DL subframe at a specific instance in time while another cell has an UL subframe. In order to avoid missing any transmission and reception opportunities, a mobile terminal may be configured to provide for simultaneous reception and transmission so as to accommodate overlapping subframes.
The TDD UL/DL subframe configuration of a cell normally defines its hybrid automatic repeat request (HARQ) timing and reception. For example, FIG. 1 illustrates a primary cell having TDD configuration #4 and a secondary cell having TDD configuration #2. The uplink ACK/NACK feedback may be provided by each of the primary and secondary cells based upon their respective configurations. In an instance in which the secondary cell supports a physical uplink control channel (PUCCH), uplink frame #8 of the secondary cell may support transmission of an ACK/NACK bit. Additionally, in downlink subframes #6, 7 and 8 of the primary cell, physical downlink shared channel (PDSCH) may be supported since simultaneous transmission and reception is allowed by the mobile terminal. Further, in uplink subframes #3 of the primary and secondary cells, the PUCCH may be from the primary cell.
By way of example, FIG. 1 illustrates a situation in which the primary and secondary cells are configured to have TDD UL/DL configurations #4 and #1 respectively. In accordance with the UL ACK/NACK timing specified by the LTE specification, a number of DL subframes require UL ACK/NACK feedback in UL subframe #3. Thus the size of the DL association set for the primary cell may be four, that is, downlink subframes #6, 7, 8 and 9, and for the the secondary cell may be one, that is, downlink subframe #9. Similarly, in an instance in which the downlink pilot time slot (DwPTS) does not support PDSCH transmissions, the size of the DL association set for UL subframe #2 would be 3 and 1 for the primary cell and the secondary cell, respectively. By having different numbers of downlink subframes that require UL ACK/NACK feedback in a respective UL subframe, the primary and secondary cells may be unbalanced.
In an instance in which a mobile terminal is configured to support PUCCH format 1b with channel selection mode b, that is, the PUCCH mode with ACK/NACK time domain bundling, the ACK/NACK bits may be compressed by mapping the ACK/NACK bits to respective states. In this regard, Table 1 presented below defines the mapping between various combinations of four DL subframes, that is, a DL association set of size 4, and respective states. In this table, A represents ACK, N represents NACK, D represents discontinuous transmission (DTX) and “any” means that the states have to be reported to or by the mobile terminal regardless of whether the mobile terminal correctly received the corresponding state.
TABLE 1
As shown by the dashed blocks in Table 1, there will be DL throughput (TP) loss due to unnecessary retransmission for those states that correspond to DL subframes designated as “any” since an “any” subframe always results in retransmission for the PDSCH.
The unbalanced scenario depicted in FIG. 1 having component carrier-specific TDD configurations may be required to co-exist with time division synchronous code division multiple access (TD-SCDMA) in one of the bands. In this regard, the co-existence with TD-SCDMA will require TDD UL/DL configuration #1 or #2 in the relative frequency band. As shown in FIG. 1, the secondary cell has a TDD configuration #1, but the other component carrier, that is, the primary cell, is configured with a TDD UL/DL configuration #4 for supporting improved downlink throughput in the cell. As such, the scenario depicted by FIG. 1 may occur with some frequency. As an example, if the PDCCH/PDSCH decoding performance is independent among the DL subframes and the ACK probability of a PDSCH is around 0.1 while the PDCCH DRX probability is much lower, then the probability of the state “A, A, N/D, any” is about 8.1 percent and the probability of the state “N, any, any, any” is about 10 percent. Thus, the DL TP loss that results from the imbalance in the DL association set sizes of two cells may occur more frequently than desired.